Lead-free solder has been growingly demanded these days from the viewpoint of environmental protection, because lead (Pb) is possibly harmful to humans. The lead-free solder, however, has generally a higher melting point than that of tin- and lead-based conventional solder. Thus the technique about solder joint structure using zinc-based lead-free solder has been developed because a melting point of the zinc-based lead-free solder can be approximated to that of tin- and lead-based solder.
First, a solder-joint technique using conventional zinc-based lead-free solder is described with reference to FIGS. 17-19 (e.g. refer to Japanese Patented Publishing No. JP9-174278). FIG. 17 shows a sectional view of a conventional solder joint structure. In FIG. 17, printed circuit board (PCB) 1 has soldering pad 2 (pad) formed of copper foil etched at a predetermined place thereon. Hole 3 extending through both PCB 1 and pad 2 is formed at a substantial center of pad 2. For instance, lead 5 of component 4, such as an electrolytic capacitor, extends through hole 3. Lead 5 is connected to pad 2 with a layer formed of lead-free and tin-, silver-, copper-based solder 6 as well as another layer formed of tin-, zing-based lead-free solder 7.
Next, steps of a conventional soldering are described hereinafter. FIG. 18 shows a flowchart illustrating steps of forming a conventional solder joint structure. FIG. 19 shows a schematic and enlarged sectional view of an essential part of the conventional solder joint structure. In FIG. 18, application step S10 starts the soldering operation by applying paste or cream of tin-, silver- and copper-based lead-free solder to PCB 1. In this application step S10, tin-, silver- and copper-based lead-free solder is printed on PCB 1 using a screen mask having 120 μm thickness.
Next, first reflow step S11 follows step S10. In step S11, lead-free solder applied to PCB 1 in step 10 is heated and melted to form solder layer 6, and as shown in FIG. 19, tin-copper alloy layer 21 of approx. 3 mm thick made from Cu3Sn alloy and Cu6Sn5 alloy is formed between pad 2 and solder layer 6. Then application step S12 follows, where paste of tin- and zinc-based lead-free solder is applied onto solder layer 6. In step S12, the paste of zinc-based lead-free solder is printed on PCB 1 by metal plate of 1.2 mm thick.
Then component insertion step S13 follows, where lead 5 of component 4 is inserted into hole 3 of PCB 1 where the paste of zinc-based lead-free solder is printed. Second reflow step S14 then follows, where the tin-, zinc-based lead-free solder is melted to form tin-, zinc-based solder layer 7, thereby component 4 is soldered to PCB 1 at its lead 5.
A temperature in second reflow step S14 should be set as low as possible so that the temperature of component 4 cannot exceed its compensation temperature during step S14. In second reflow step S14, the component is thus soldered at the lowest possible temperature for melting tin-, zinc-based lead-free solder, so that the temperature of tin-, zinc-based lead-free solder has never exceeded the liquidus temperature in step S14.
Because of what is discussed above, upon formation of tin-, zinc-based lead-free solder layer 7, it is considered that tin-zinc alloy layer 22 as shown in FIG. 19 is formed between tin-copper alloy layer 21 and lead-free solder layer 6 formed in first reflow step S11. Actually, the presence of layer 22 is confirmed. In such a conventional solder joint structure, the zinc in solder layer 7 and the copper in pad 2 diffuse and enter into solder layer 6, thereby forming copper-zinc alloy layer 22 on top of tin-copper alloy layer 21. If this state undergoes a high temperature environment, zinc and copper diffusing in layer 6 are further consumed, so that alloy layer 22 grows gradually for hours. The schematic sectional view shown in FIG. 19 shows boundaries explicitly between respective layers; however, those boundaries are prepared for simplifying the description. Actually, each component ratio varies time-dependently and such explicit boundaries do not exist. In this description, however, the boundaries are shown in order to illustrate ranges recognizable with SEM pictures as the layers.
Growth of tin-zinc alloy layer 22 weakens the strength of joint structure, so that the reliability of the soldering lowers, and this is the problem to be solved of the conventional soldering.
The present invention addresses the foregoing problem, and aims to provide a solder-joint technique using zinc-based lead-free solder which resists lowering the joint strength of soldering.